2Metabolic Regulation Genetic Level Regulation: Control which protein is synthesized through adjusting the rate of transcription of that gene:Feedback repression: The of enzymatic activity accumulates and blocks tFor repression, the is required which can bind to the operator region and hinder RNA polymerase binding.The repressor protein can block transcription when bound to the (typically the end product of the pathway).
4Genetic Organization of the Tryptophan Operon DNA templateencoding related enzymes for tryptophan synthesisfrom chorismate.encoding repressorL. Stryer, Biochemistry, p.813. P.586.show chorismate.Only when the repressor binds with tryptophan, it can bindon the operator region and block the transcription.Operon: In prokaryotes, a set of genes, encoding proteins with related functions, under the control of a single promoter-operator.
5Metabolic regulation Genetic Level Regulation: Induction: a ( often a substrate for a pathway) accumulates and acts as an inducer of transcription.The inducer will bind the repressor protein, and the complex is inactive as a repressor.
7ExampleInducer: allolactose modified from lactose in the cell.
8e.g. The lactose operon controls the synthesis of three proteins (Lac z (lactase), lac y, lac a ) involved in lactose utilization as a carbon and energy source in E. coli.Lac iPromoterOperatorLac zLac yLac aRNA polymerasem-RNABiochemicstry, Lumbert StryerLac z: encode beta-galatosidase, lactase, which cleaves the disaccharides loctose to glucoae and galatose.Lac y: galatoside permease, trnasport of lactose acroos the cell membrane.Lac a: thiogalactoside transacetylase, it is unessential for lactose metabolism, but tansfer the acetyl from acetyl CoA to C-6 HYDROXYL GROUP OF A THIOGALACTOSIDE.repressorLac i encoding repressor.allolactose
9Catabolite Repression (Glucose Effect) Inducer: allolactose modified from lactose in the cell.Induction of allolactose might not be sufficient for maximum transcription if a carbon-energy source (e.g. glucose) preferred to lactose is present.Only when glucose is depleted, the cell will expend energy to create a pathway to utilize the less favorable carbon-energy source lactose.
10Metabolic Regulation Catabolite Repression (glucose effect) When the cell has an energetically favorable carbon-energy source (e.g. glucose) available,it will not expend significant energy to create a pathway for utilization of a less favorable carbon-energy source;it will not transcript the related enzyme for such reaction.
11Metabolic Regulation Genetic Level Regulation: Some genes are regulated.Others are not (constitutive):their gene products are made at a relatively constant rate irrespective of changes in growth conditions.( enzymes are expected to use under almost any conditions such as that involved in glycolysis)
12Metabolic RegulationCellular Level Regulation - Metabolic Pathway Control:The can be controlled by enzyme activity.The activity of allosteric enzymes can be controlled by effectors including inhibitors and activators.Most often the reaction in the pathway is inhibited by accumulation of : feedback inhibition or end-product inhibition.
14What are the differences between feedback repression and feedback inhibition?
15Feedback inhibition Feedback repression Regulation level Genetic: RNA transcriptionCellular:Activity of enzymeComplex formedEnd product +repressorEnd product + enzymeEffectOperator on DNA template occupied by the complexReduced enzyme activityConsequenceBlocked TranscriptionThe respective reaction is inhibited.
16Metabolic Regulation Cellular level- metabolic pathway controls: The activities of a group of enzymes (pathway) can be controlled.- Isozymes- Concerted feedback- Sequential feedback- Cumulative feedbackPlease refer to the textbook p.123.
17Metabolic RegulationCellular level- metabolic pathway controls through:IsozymesA number of separate enzymes initially carry out the same conversion, each of which is sensitive to inhibition by a different end product.
18The common pathway leading to the synthesis of the aromatic amino acids contains three isozymes. Each of these enzymes is specifically feedback-inhibited by one of the aromatic amino acids. Note how an excess of all three amino acids is required to completely shut off the synthesis of DAHP.
19Metabolic Regulation - Concerted feedback inhibition More than one end product or all end products must be present in excess to repress the first enzyme.
21Metabolic Regulation - Sequential feedback inhibition The common steps are inhibited by the product before the branch, and the first enzyme of each branch is inhibited by the branch product.High levels of P1 and P2 inhibit enzyme E3 and E4, respectively → M3 will accumulate →the pathway is inactivated if both P1 and P2 are high.M1M2M3M4M5P1P2XE3E1E2E4
23Metabolic Regulation- Cumulative feedback inhibition or cooperative feedback inhibition- A single allosteric enzyme may have effector sites for - several end products of a pathway;- each effector causes only partial inhibition.- Full inhibition is a cumulative effect.CumulativeConcerted